1. Field of the Invention
The invention relates generally to a controllable liquid-crystal display device and more particularly to a display device including a liquid crystal contained between transparent electrodes located on two plane surfaces, between a first transparent support plate and a second support plate and between a first polarizer and a second polarizer combined with a reflector.
2. Description of the Prior Art
Such controllable liquid-crystal display devices are already known from DOS No. 2,158,563.
In such known display devices the incident light must pass twice through the polarizers, the support plates and the liquid crystal. In the bright position, therefore, (transmitting directions of both polarizers are perpendicular to each other when using a nematic liquid crystal with a 90.degree. helical structure) the intensity of the outgoing light is considerably weaker than the incoming, non-polarized light. With the usual linear polarizers, however, an arbitrarily high transmissivity cannot be achieved, because even in the ideal case only light oscillating in a certain direction is transmitted, while the polarizer absorbs the light oscillating in other directions. In the case of commercially available polarizers, polarizers with the greatest transmissivity show the lowest extinction ratio, where by extinction ratio is meant the relationship of the intensity of light passing through a pair of polarizers with parallel axes to that passing through a polarizer pair with crossed axes. Under unfavorable lighting conditions, therefore, the case of reading a liquid-crystal display device operating by reflection can be severely impaired.
The commercial polarizer known as the Polaroid HN 42 for non-polarized incident light, for example, has a transmissivity of 35% for two polarizers in the bright position, and a maximum attainable extinction ratio of 1 : 70. In the case of a display with a bright background, therefore, the brightness of the background is equivalent to less than 35% of the incident light because the reflector is often made diffusive to avoid undesired reflection of the light source, and the light when reflected back has to pass through the display again. For this reason, the display is difficult to read under conditions of low ambient lighting level, even when the contrast between the electrically energized characters and the background is adequate. An improvement in legibility by brightening the background can be achieved with a Polaroid HN 55 polarizer pair, but contrast is then much poorer.